MGB Register Forum

A friend informs me there are two different thicknesses of flanges at the cylinder head on the EXHAUST manifolds . He is asking if there are also two thicknesses on INLET manifolds . Currently he has a thin flange on an exhaust manifold and a thick one on an inlet manifold and is keen to match them up . Thanks for any help you can give.

I've never had to change / replace either of my inlet or exhaust manifolds. However a quick look at Moss's catalogue implies there was only one type of inlet manifold (CHM171) shown as NCA , but two exhaust manifolds -1962 - 70 (12H709R) & 1970 - 80 (12H3911R) . I don't know if this explains the reason for the difference in flange thicknesses or perhaps differences in earlier or later cylinder heads.
Cheers
Charles

Ray, I have some inlet and exhaust manifolds which I can measure for you if that would help. I also have the original manifolds on my '72 BGT which I can measure.

Ian F

Ian. That would be a help , Ian , thanks . My pal tells me that the exhaust flanges measure 9/16 th and 3/8 th inches so he is curious to know whether inlet manifolds are similar . Ray

I have measured my spares (off the car) which seem the same as the ones fitted to my 72 BGT (which are difficult to measure of course).
The measurements differ from what you have indicated.

Inlet: 12H2368 (?). is 0.445 inches thick.
Exhaust: 12H3911 is 0.445 inches thick

These measurements are midway between yours, and equate to circa 7/16". An allowance must be made for corrosion and dirt on both items!

Ian F

Ian. Thank you for going to the trouble of measuring the manifolds you have . They are indeed quite different to the sizes l have been given so l will pass your measurements on to my friend . If anybody else can provide any other measurements it will be interesting to see what they are .
Thanks again Ian and also thanks to Charles for his reply .

Ray, it did occur to me that the exhaust manifold with the thin flange might be an aftermarket tubular manifold? Are you sure it is the standard cast iron job?
Just a thought!

Ian F

Hi
Just measured the original exhaust manifold from my 74 B with a Vernier caliper. The average, after measuring across all flanges is 12 mm or 0.47”. 15/32” (According to my Zeus tables). I can’t measure the inlet manifold as its fitted to the car with a 3 branch manifold but would assume the flanges are of a similar thickness to the original exhaust manifold otherwise it would not clamp up correctly.
Hope this helps.
Mike Barclay

Thank you , Michael, for your exhaust flange measurement. It is within a cats whisker of that give by Ian so l think that confirms it as the original size .
Ian , yes he does have a tubular manifold which has a 3/8 th flange which he says is the later flange 12H3911 thickness . However , your flange of that number measured 0.445 inches . Very confusing .
I will present him with all your replies and leave him to it !.
Thank you all for your trouble .

the answer to your question is yes there are.

I have one of each in the shed but will need to root them out for part numbers. Don't know when switch was made.